1. Field of the Invention
The present invention relates to an exhaust gas purification system of an internal combustion engine having a particulate filter. Specifically, the present invention relates to a technology for regenerating the particulate filter at appropriate timing.
2. Description of Related Art
In recent years, improvement of exhaust emission of internal combustion engines mounted to automobiles has been required. Specifically, in a compression ignition diesel engine, of which fuel is light oil, exhaust particulate matters contained in exhaust gas, such as soot or soluble organic fractions, should be reduced in addition to gasses of carbon monoxide, hydrocarbon, and nitrogen oxides. Therefore, a particulate filter is disposed in an exhaust passage for collecting the exhaust particulate matters contained in the exhaust gas.
If the exhaust gas enters the particulate filter, the exhaust gas passes through porous partition walls in the particulate filter. At that time, the exhaust particulate matters contained in the exhaust gas are collected at surfaces or pores of the partition walls. If a quantity of the collected and deposited particulate matters (a deposition quantity) increases excessively, flow resistance in the particulate filter increases and a back pressure of the engine increases. As a result, an output of the engine will be deteriorated. Therefore, the particulate filter is regenerated by reducing the exhaust particulate matters collected in the particulate filter at appropriate timing. A system capable of regenerating the particulate filter during operation of the engine uses oxidizing properties of an oxidation catalyst such as platinum disposed on the particulate filter.
As modes of the regeneration of the particulate filter for combusting and reducing the deposited exhaust particulate matters, there are a mode of spontaneous regeneration and a mode of compulsory regeneration. In the spontaneous regeneration mode, the deposited exhaust particulate matters are combusted and reduced spontaneously by heat of the exhaust gas, of which temperature is increased to a high temperature when a vehicle driver presses down an accelerator and a fuel injection quantity increases, for instance. In the compulsory regeneration mode, the deposited exhaust particulate matters are combusted and reduced compulsorily. For instance, in the compulsory regeneration, a post-injection for injecting fuel in an exhaustion stroke is performed to supply the fuel to the particulate filter. Thus, the deposited exhaust particulate matters, which are more difficult to oxidize than the injected fuel, are oxidized and reduced with the use of heat generated by combusting the supplied fuel. The temperature of the exhaust gas can also be increased by retarding fuel injection timing.
The spontaneous regeneration occurs in accordance with accelerator manipulation of the vehicle driver. Therefore, the spontaneous regeneration occurs on an irregular basis. The compulsory regeneration can be performed anytime.
If the compulsory regeneration is performed frequently, a fuel cost will increase. If a time interval to the next generation is too long, the quantity of the deposited exhaust particulate matters will increase excessively. In such a case, the exhaust particulate matters will be combusted rapidly in the next regeneration and the particulate filter will be heated to an abnormally high temperature. There is a possibility that the particulate filter is damaged. Therefore, the regeneration timing should be preferably determined by calculating the deposition quantity of the exhaust particulate matters based on operating states of the engine. The flow resistance increases as the deposition quantity of the exhaust particulate matters in the particulate filter increases. A differential pressure between an inlet and an outlet of the particulate filter increases as the flow resistance increases. Therefore, a system disclosed in JP-A-H07-332065 (Patent Document 1) senses the differential pressure and determines that the regeneration timing is reached if the sensed differential pressure exceeds a predetermined value.
A combustion speed of the deposited exhaust particulate matters is not necessarily even in the particulate filter. There is a possibility that a variation in the deposition in the particulate filter increases because of the repetition of the spontaneous regeneration and the deposition if the combustion speed is uneven in the particulate filter. Moreover, in the compulsory regeneration, regeneration efficiency decreases as the regeneration progresses, or as a quantity of remaining exhaust particulate matters decreases and the deposition quantity decreases. Therefore, the deposited exhaust particulate matters should be preferably eliminated, or the deposition quantity should be preferably decreased to zero, only in one compulsory regeneration operation while multiple compulsory regeneration operations are performed. The other compulsory regeneration operations should be preferably ended in a state in which the exhaust particulate matters remain in the particulate filter. However, in this scheme, there is a possibility that a deposition distribution inside the particulate filter enlarges further. The system of Patent Document 1 can grasp an average deposition state, but cannot grasp the deposition distribution. Therefore, there is a possibility that the rapid combustion occurs in some portions in the particulate filter.